Optimum integration of fischer-tropsch synthesis and syngas production

ABSTRACT

A method is described for conversion of natural gas or other fossil fuels to higher hydrocarbons, comprising the following steps: a) reaction of natural gas with steam and oxygenic gas in at least one reforming zone in order to produce a synthesis gas consisting primarily of H 2  and CO, in addition to some CO 2 ; b) passing said synthesis gas to a Fisher-Tropsch reactor in order to produce a crude synthesis stream consisting of lower hydrocarbons, water and non-converted synthesis gas; c) separation of said crude synthesis stream in a recovery zone, into a crude product stream mainly containing heavier hydrocarbons, a water stream and a tail gas stream mainly containing the remaining constituents; which is charaterised in that the method also comprises the following steps; d) stream reformation of at least part of the tail gas in a separate steam reformer. e) introduction of the reformed tail gas into the gas stream before this is led into the Fischer-Tropsch reactor.

[0001] The present invention regards a system for chemical conversion ofnatural gas or another suitable fossil fuel to synthetic hydrocarbons(syncrude). In particular, the present invention regards a system foroptimising the production of synthetic hydrocarbons.

[0002] Known processes for conversion of natural gas or other fossilfuels to synthetic hydrocarbons comprise two steps. First, the naturalgas or other fossil fuel is converted to synthesis gas, i.e. a mixtureconsisting predominantly of hydrogen and carbon monoxide, as well assome CO₂, which in a second step is converted to synthetic hydrocarbonsthrough the so-called Fischer-Tropsch (FT) synthesis. The synthetichydrocarbon product normally consists of higher hydrocarbons, i.e.pentane and higher compounds (C₅₊). The process may also include anadditional step in which the synthetic hydrocarbon crude product isupgraded to final products.

[0003] Synthesis gas for production of synthetic hydrocarbons isnormally produced by steam reforming or partial combustion, or acombination of these two reactions. The water gas shift reaction alsoplays an important part in the production of synthesis gas. Thesereactions may be written as follows: 1) steam CH₄ + H₂O = CO + 3H₂ ΔH =206 kJ/mole reforming 2) partial CH₄ + {fraction (3/2)}O₂ = CO + 2H₂O ΔH= −519 kJ/mole combustion 3) water CO + H₂O = CO₂ + H₂ ΔH = −41 kJ/molegas shift

[0004] The Fischer-Tropsch synthesis for producing synthetichydrocarbons may be written as follows:

FT synthesis CO+2H2=[—CH₂—]+H₂O ΔH=−167 kJ/mole  4)

[0005] where [—CH₂—] is the basic building block for the hydrocarbonmolecules. The FT synthesis is highly exothermic, which leads to heattransfer being a significant factor in the design of an FT reactor.

[0006] An important parameter for determining the theoretical maximumyield of synthetic hydrocarbons is the stochiometric number SN, definedas:

SN=(H₂—CO₂)/(CO+CO₂)  5)

[0007] Theoretically, the yield of synthetic hydrocarbons is at itshighest when SN=2.0 and CO does not react further to form CO₂ via thewater gas shift reaction (equation 3). In this case, the H₂/CO ratiowill be equal to SN, i.e. 2.0, which theoretically gives the highestyield of synthetic hydrocarbons in accordance with equation 4. Inpractice however, the production of synthesis gas will always involvethe water gas shift reaction to a certain degree, so that the CO yield,and thus also the synthetic hydrocarbon yield, becomes somewhat lower.

[0008] Further, the maximum yield of synthetic hydrocarbons is inreality achieved at a somewhat lower H₂/CO ratio, typically around1.6-1.8. At an H₂/CO ratio of 2.0 or more, the synthetic hydrocarbonyield will be reduced due to the formation of more methane and otherlower hydrocarbons (C₄), which are normally undesirable products.

[0009] The preferred technology for producing synthetic hydrocarbonsfrom synthesis gas is non-catalytic partial oxidation (POX) orautothermal reforming (ATR), in which partial combustion is combinedwith adiabatic catalytic steam reforming (equation 1) in the samereactor unit.

[0010] Another technology is combined reforming with a tubular catalyticsteam reformer followed by an ATR.

[0011] A desired H₂/CO ratio is achieved by running the synthesis gasreactor with a combination of a low steam/carbon ratio (S/C) and a hightemperature, in addition to recirculating part of the CO₂-rich tail gasfrom the FT synthesis to the synthesis gas reactor in order to limit thewater gas shift activity (equation 3). In this manner, the H₂/CO ratiowill approach the achieved value of SN.

[0012] The drawback of the known techniques for producing synthetichydrocarbons is low carbon efficiency in comparison with the theoreticalachievement. The carbon efficiency is defined as the relationshipbetween the total amount of carbon in the produced crude product ofsynthetic hydrocarbons and the total amount of carbon in the natural gasfeed. As such, the carbon efficiency is a measure of how much of thecarbon in the feed that actually ends up in the final product, and howmuch ends up as CO₂. A plant with low carbon efficiency gives a lowproduct yield, a large CO₂ emission and thus an environmental problem.

[0013] As mentioned, catalytic autothermal reforming (ATR) andnon-catalytic partial oxidation (POX) are the preferred technologies forproduction of synthesis gas for the FT synthesis. By using natural gasas a feed, these technologies produce a synthesis gas with an SN valuetypically in the range 1.6 to 1.8, which gives the highest yield ofsynthetic hydrocarbons locally in the FT reactor. However the SN valueis lower than 2.0, which for the plant as a whole implies a lower carbonefficiency than that which may theoretically be achieved, due to ahydrogen deficiency.

[0014] Combined reforming, which normally takes place in a tubularcatalytic steam reformer followed by a secondary reformer with an oxygenfeed, is capable of producing synthesis gas with an SN value of 2.0,which should theoretically give the highest carbon efficiency in theplant for production of synthetic hydrocarbons. The real carbonefficiency will however not be higher than that which is achieved by useof POX or ATR, due to the higher degree of recirculation of tail gas tothe synthesis reaction that is required in order to restrict a greaterwater gas shift activity than in the ATR as a result of the higher S/Cratio, and due to a lower yield of the desired higher synthetichydrocarbons at this SN value.

[0015] It is thus an object of the present invention to provide animproved method for conversion of natural gas or other fossil fuels tohigher hydrocarbons, in which the above mentioned drawbacks of the knowntechniques have been overcome.

[0016] According to the present invention, this is achieved by a methodfor conversion of natural gas or other fossil fuels to higherhydrocarbons, which comprises the steps of:

[0017] a) reacting natural gas with steam and oxygenic gas in at leastone reforming zone in order to produce a synthesis gas that consistsprimarily of H₂ and CO, in addition to some CO₂;

[0018] b) lead said synthesis gas to a Fischer-Tropsch reactor in orderto produce a crude synthesis stream consisting of lower hydrocarbons,higher hydrocarbons, water, and unconverted synthesis gas;

[0019] c) separating said crude synthesis stream in a recovery zone,into a crude product stream that primarily contains higher hydrocarbons,a water stream and a tail gas stream that mainly contains the remainingconstituents;

[0020] characterised in that the method also comprises the steps of;

[0021] d) steam reforming at least part of the tail gas in a separatesteam reformer;

[0022] e) introducing the reformed tail gas into the gas stream beforethis is fed into the Fischer-Tropsch reactor.

[0023] “Lower hydrocarbons” refers to C₁-C₄ hydrocarbons. “Higherhydrocarbons” refers to C₅₊ hydrocarbons.

[0024] It is preferable for the steam reforming in step d) to take placeat conditions that favour the conversion of CO₂ to CO by reversiblewater gas shift reaction.

[0025] Moreover, it is preferable to also hydrogenate that part of thetail gas that is steam reformed, in order to saturate any unsaturatedhydrocarbons prior to step d).

[0026] In a preferred embodiment, natural gas is fed to the steamreformer in step d) together with the tail gas feed.

[0027] In a preferred embodiment, the reformed tail gas is introducedinto the gas stream after step a), but before step b).

[0028] In another preferred embodiment, the reformed tail gas isintroduced into the gas stream before step a).

[0029] It is also preferred that part of the reformed tail gas beintroduced into the gas stream before step a) and part of it beintroduced after step a) but before step b).

[0030] Use of the present method has several advantages over previouslyknown techniques.

[0031] By reforming and recirculating the tail gas, it becomes possibleto:

[0032] Increase the SN value from typically 1.6-1.8 for an ATR toapproximately 2.0.

[0033] Maintain or increase the CO yield, so that the H₂/CO ratioapproaches the SN value.

[0034] Achieve an H₂/CO ratio of less than 2.0 locally at the inlet tothe FT reactor, which give a higher yield of higher hydrocarbons.

[0035] The present method results in higher carbon efficiency and higherthermal efficiency. This gives a desired reduction in the CO₂ emission,which is desirable, both for environmental and economic reasons. Theoxygen consumption by the present method is lower than in the case ofconventional plants for production of synthesis gas by use of POX orATR, which entails reduced capital costs and lower power consumption.

[0036] It is also possible to achieve operational benefits such asincreased stability by the oxygen fired synthesis gas reactor operatingat a somewhat lower output temperature than that which is the case whenusing previously known technology. The increased methane content (lowerconversion of natural gas) caused by this will be reformed in the tailgas reformer.

[0037] By eliminating the recirculation of tail gas to the main sectionfor synthesis gas, it is also possible to economise with regard to thesize of the equipment, and thereby to save costs in this section.

[0038] The invention will now be explained in greater detail withreference to the accompanying drawings, in which:

[0039]FIG. 1 is a simplified flow diagram showing the process forproducing synthetic hydrocarbons by the present method;

[0040]FIG. 2 is a more detailed flow diagram showing a first preferredembodiment of the present method; and

[0041]FIG. 3 is a more detailed flow diagram showing a second preferredembodiment of the present method.

[0042] The simplified flow diagram in FIG. 1 shows a method forproducing synthetic hydrocarbons by using natural gas as the main sourceof carbon and hydrogen, while FIGS. 2 and 3 represent more detailed flowdiagrams showing two preferred versions of this method.

[0043] The present method of FT synthesis based on natural gas or otherfossil fuels may be divided into three main parts; that is a first partfor production of synthesis gas, a second part for Fischer-Tropschsynthesis (FT synthesis) and a third part for reforming tail gas fromthe FT synthesis.

[0044] Production of Synthesis Gas

[0045] Natural gas enters the plant primarily through natural gas line1. The natural gas is first heated to typically about 350-400° C. beforebeing passed through a desulphurization unit 20. Here sulphur, which ispresent in the natural gas in the form of various organic compounds, isconverted to hydrogen sulphide through contacting it with an appropriatehydrogenation catalyst. The hydrogen sulphide is then reduced to adesirable level by use of a zinc oxide layer.

[0046] After desulphurization, water vapour is added to the gas in orderto ensure a desired ratio between water vapour and carbon (S/C ratio),typically from about 0.6 to 1.3 for production of synthetichydrocarbons. The gas/water vapour mixture is preheated and introducedinto a prereformer 3 that converts C₂ and higher hydrocarbons to CH₄, COand CO₂. The operating temperature in the prereformer 3 is typically inthe range 430 to 500° C. The prereformer may be omitted, in particularwhen using natural gas with a low content of C₂₊.

[0047] Hydrogen, which is required in the desulphurization unit 20 andin the prereformer 3, is added to the natural gas before it enters thedesulphurization unit 20. As indicated in the figures, part of the tailgas containing amongst other things hydrogen, may be recirculated andadded to the gas before it enters the desulphurization unit 20. It isalso possible to recover hydrogen from said tail gas by e.g. pressureswing adsorption (PSA), or hydrogen may be supplied from another source.

[0048] The prereformed gas mixture is then heated further to atemperature of typically 550-650° C., before being sent into anautothermal reformer (ATR) 5 together with oxygen or an oxygenic gassuch as e.g. air, which comes in through an oxygen inlet 4, normallyfrom a cryogenic oxygen plant (not shown). The gas that is fed to ATR 5is converted to synthesis gas in ATR 5 through partial combustion in theupper part of ATR 5 and steam reforming of the gases across a nickelcatalyst in the lower section of ATR 5. The formation of synthesis gasin ATR 5 typically takes place at a pressure of about 30-40 bar, and theoutlet temperature of the gas from ATR 5 is typically in the range950-1050° C.

[0049] The hot synthesis gas leaving ATR 5 in synthesis gas line 6 isfirst cooled in a heat exchanger 22, in which typically water from inlet21 is converted to high pressure steam in outlet 23. One heat exchangerhas been indicated in the figures, however in practice there may be aplurality of heat exchangers connected in series, cooling the synthesisgas to the desired temperature. The last cooling down to typically40-70° C. is achieved by use of cooling water.

[0050] Condensed water is then separated out from the synthesis gasbefore this is led to a Fischer-Tropsch synthesis reactor 7.

[0051] Fischer-Tropsch Synthesis

[0052] The desired synthetic hydrocarbons are formed in a known mannerin a Fischer-Tropsch reactor (FT reactor) 7 in which hydrogen and carbonmonoxide are converted to higher hydrocarbons, leaving water as aby-product, according to equation (4) above. The FT reactor 7 istypically run at 20-40 bar pressure and a temperature of 180-240° C. Asthe reaction is exothermic, heat is normally removed from the reactor 7through generation of water vapour at an intermediate pressure oftypically around 5-20 bar.

[0053] The product streams from the FT reactor 7 typically contain thedesired product in the form of C₅₊ hydrocarbons, by-products in the formof lower hydrocarbons (C⁵⁻), CO₂ and water, as well as non-reactedsynthesis gas, i.e. CO and hydrogen. This product stream is separated ina product recovery unit 24, into a crude product stream containingprimarily the desired hydrocarbon product in outlet 25, separated waterin outlet 26 and a tail gas stream chiefly comprising the aboveby-products and non-reacted synthesis gas, in tail gas line 9.

[0054] The tail gas in tail gas line 9 is in turn split into three. Afirst part goes through recirculation line 10 and is compressed in acompressor 27 for recirculation to the synthesis gas production asindicated below, a second part goes through a reforming line 12 to atail gas reforming process, while a third part is drawn off throughbleed line 11 and, if so required, used as fuel in heat consuming partsof the process.

[0055] Tail Gas Reforming

[0056] The tail gas in tail gas line 12 is preferably first led to atail gas hydrogenator 28 in order to saturate any unsaturatedhydrocarbons. The operating temperature of the hydrogenator 28 istypically 220-250° C., while the operating pressure is around 20-40 bar.This tail gas hydrogenator 28 is not obligatory preferred, howeverunsaturated hydrocarbons have a greater tendency towards coking thansaturated hydrocarbons during the subsequent high temperature treatment.

[0057] After the tail gas hydrogenator 28, water vapour and possibly anamount of natural gas are added to the tail gas in vapour inlet 13 andgas inlet 14 respectively, before the gas is preheated and passed into atail gas reformer 15 in which light hydrocarbons are steam reformed onformation of CO and hydrogen, cf. equation 1) above, while CO₂ presentin the tail gas is converted to CO through a reverse water gas shiftreaction according to equation 3). The natural gas feed can be takenfrom the product stream from the prereformer 3 (clean split).

[0058] The operating temperature of the tail gas reformer is typicallyabove 800° C., preferably from 850 to 950° C., while the operatingpressure is normally from 10 to 40 bar. If so necessitated by theoperating pressure difference between the tail gas reformer and the FTreactor, a compressor may be provided downstream of the tail gasreformer. Energy for these reactions can be provided by combustion offuel that may consist of a small part of the tail gas from bleed line11.

[0059] Depending on the C²⁺ content of the gas that may be added in gasinlet 14, it may become necessary to install a prereformer after theaddition of water vapour, upstream of the tail gas reformer. The purposeof such a prereformer, which is of the same type as the prereformer 3,is to convert ethane and higher hydrocarbons in the gas stream tomethane, CO and CO₂, thereby to avoid/reduce coking at hightemperatures. If no natural gas is added in inlet 14, or when usingnatural gas with a methane content of 90% or more, there will normallynot be a requirement for a prereformer here.

[0060] The hot flow of reformed tail gas from the tail gas reformer 15can then be cooled in a heat exchanger 30 in which water that comes inthrough inlet 31 is converted to water vapour that exits through vapouroutlet 32. One heat exchanger has been indicated in the figures, howeverin practice there may be a plurality of heat exchangers connected inseries, cooling the synthesis gas to the desired temperature. Condensedwater is then separated out from the reformed tail gas before this iscompressed in compressor 33 and led through tail gas line 16 tosynthesis gas line 6 before this enters the FT reactor. It is alsopossible to introduce the reformed tail gas directly into the gas streambetween the prereformer 3 and the autothermal reformer (ATR) 5. Inaddition it will be possible to split the flow of reformed tail gas andlead one component stream to the FT reactor 7 and one component streamto ATR 5.

[0061] The purpose of leading the reformed tail gas to ATR 5 is toachieve further steam reforming and the formation of CO through thereversible water gas shift reaction, as the temperature of ATR 5 ishigher than that of the tail gas reformer, thus attaining a highercarbon efficiency for the plant. This effect may be partially counteredthrough combustion of CO and hydrogen to CO₂ and water. The choice ofsolution here, and any decision regarding how much of the reformed tailgas goes where, will depend on a number of operational parameters.

[0062] The primary purpose of reforming and recirculating tail gasaccording to the present invention is to steam reform lower hydrocarbonsto CO and hydrogen, thereby to increase the stochiometric number SNtowards the desired value of 2.0, which is an important condition forachieving a significantly higher efficiency for the process plant. Asthe tail gas contains little in the way of light hydrocarbons, steamreforming of this stream alone will only give a limited increase inefficiency. Adding natural gas or another source of lower hydrocarbonsthrough gas inlet 14 will therefore give a further increase in carbonefficiency.

[0063] Another advantage of adding natural gas to the tail gas reformeris to reduce the amount of feed gas to ATR 5, which gives a lower oxygenconsumption than that of a conventional synthesis plant with ATR.

[0064] The Overall System

[0065] In total, the present method gives a noticeable and importantincrease in the carbon efficiency, a reduction of the oxygen consumptionand improved overall economy for the plant.

[0066] By reforming and recirculating a significant portion of the tailgas to the FT reactor 7 and/or ATR 5, the equipment in the feed sectionto the ATR unit can be smaller than that which would be the case if thetail gas were to be recirculated to the hydrogenation unit 28, as iscommon today.

[0067] The tail gas from the product recovery section 24 is, asmentioned above, split into three parts. It has proven advantageous torecirculate 0-20%, for example around 10%, to the hydrogenation unit 28;use 0-40%, for example around 30% as fuel in the tail gas reformer; anduse 40-80%, for example around 60%, as feed to the tail gas reformingpart of the process.

EXAMPLE

[0068] Five different plants/modes of operation of the plant weresimulated in order to show the advantages of the present inventioncompared with previously known technology traditionally used in plantsfor synthesis of synthetic hydrocarbons. In all the examples, theproduction was set at 20 000 BPD or 101 tons/hour.

[0069] The examples were as follows:

[0070] Ex. A Production of synthetic hydrocarbons by conventionalautothermal reforming (ATR).

[0071] Ex. B Production of synthetic hydrocarbons by conventionalcombined reforming.

[0072] Ex. C Production of synthetic hydrocarbons by ATR and F-T tailgas reformer. No addition of natural gas to the tail gas reformer. Theproduct from the tail gas reformer was fed to the F-T reactor.

[0073] Ex. D Production of synthetic hydrocarbons by ATR and F-T tailgas reformer. 10% of the natural gas feed to the process is addeddirectly to the tail gas reformer. The product from the tail gasreformer was fed to the F-T reactor. The portion of the tail gas that isdrawn off from the plant is used is fuel gas in the tail gas reformer.

[0074] Ex. E Production of synthetic hydrocarbons by ATR and F-T tailgas reformer. 20% of the natural gas feed to the process is addeddirectly to the tail gas reformer. The product from the tail gasreformer was fed to ATR. 3% of the total natural gas feed is used asfuel in the tail gas reformer together with the portion of the tail gasthat is drawn off from the plant.

[0075] The crude product was natural gas with the following composition:Co₂  1,84% N₂  0,36% CH₄ 80,89% C₂H₆  9,38% C₃H₈  4,40% C₄H₁₀  2,18%C₅H₁₂  0,62% C₆H₁₄  0,22% C₈H₁₈  0,11%

[0076] These simulations gave the following results as to the mostimportant key data: Ex. A Ex. B Compar. Compar. ex. ex. Ex. C Ex. D Ex.E Natural gas feed, 7 767 7 790 7 150 7 062 7 070 kmol/h S/C, synthesis0.6 1.8 0.6 0.6 0.6 gas line Oxygen 4 590 3 289 3 801 3 399 3 290consumption, ton/day Fischer-Tropsch 40 25 30 30 30 tail gas used asfuel gas, % of total tail gas Fischer-Tropsch 60 75 9 9 9 tail gas addedto ATR, % of total tail gas Fischer-Tropsch — — 61 61 61 tail gas addedto tail gas reformer, % of total tail gas S/C³ tail — — 53 1.0 0.6 gasreformer CO₂/C³ tail gas — — 5.3 1.0 0.6 reformer Outlet temperature — —900 900 900 tail gas reformer, ° C. Carbon efficiency, 71.0 70.9 77.178.0 77.9 %¹ Thermal efficiency, 59.4 59.2 64.6 65.3 65.3 %² CO₂emission, 127.11 128.39 92.50 87.46 88.00 ton/h

[0077] The above table clearly shows the advantages of using of thepresent method (ex. C, D and E) in preference to the previously knownmethods (ex. A and B).

[0078] For the same quantity of product, the present method reduces theconsumption of natural gas by around 8-10%, which in turn is directlyreflected by the carbon efficiency and the thermal efficiency, which forthe present method are significantly higher than when using thepreviously known methods.

[0079] Another significant effect, which is clearly associated with theabove results, is that of the considerable reduction in CO₂ emissionsfor the same produced quantity of synthetic hydrocarbons. As can be seenfrom the above table, the CO₂ emissions by use of the present method arearound 40% lower than those caused by use of conventional methods.

[0080] The oxygen consumption in example B, which is a method accordingto prior art, was the lowest among the simulated examples. Although lowoxygen consumption is positive, the results for the critical parameters,i.e. carbon efficiency and thermal efficiency, are significantly poorerthan in the case of the present invention, i.e. examples C, D and E.

[0081] The above invention has been described as using natural gas asthe source of carbon. The process may however be used for all types ofgas that contain large amounts of lower hydrocarbons, as well as forother fossil fuels and possibly combinations of various carbon sources.

1. A method for conversion of natural gas or other fossil fuels tohigher hydrocarbons, comprising the following steps: a) reaction ofnatural gas with steam and oxygenic gas in at least one reforming zonein order to produce a synthesis gas consisting primarily of H₂ and CO,in addition to some CO₂; b) passing said synthesis gas to aFischer-Tropsch reactor in order to produce a crude synthesis streamconsisting of lower hydrocarbons, water and non-converted synthesis gas;c) separation of said crude synthesis stream in a recovery zone, into acrude product stream mainly containing higher hydrocarbons, a waterstream and a tail gas stream mainly containing the remainingconstituents; characterised in that the method also comprises thefollowing steps; d) steam reformation of at least part of the tail gasin a separate steam reformer; e) introduction of the reformed tail gasinto the gas stream before this is led into the Fischer-Tropsch reactor.2. Method according to claim 1, characterised in that the temperatureduring the steam reforming in step d) is above 800° C., preferably from850 to 950° C.
 3. Method according to claim 1 or 2, characterised inthat the portion of the tail gas that is steam reformed is alsohydrogenated in order to saturate any unsaturated hydrocarbons prior tostep d).
 4. Method according to one or more of the preceding claims,characterised in that natural gas is added to the steam reformer in stepd) together with the tail gas feed.
 5. Method according to one or moreof the preceding claims, characterised in that the reformed tail gas isintroduced into the gas stream after step a), but before step b). 6.Method according to one or more of claims 1 to 4, characterised in thatthe reformed tail gas is introduced into the gas stream before step a).7. Method according to one or more of claims 1 to 4, characterised inthat a portion of the reformed tail gas is introduced into the gasstream before step a), and a portion is introduced after step a) butbefore step b).